In medicine, the health of a subject is frequently monitored via the periodicity of certain biological processes. For example an electrocardiogram measures the periodic impulses of the heart in a wide variety of diagnostic tests. From this periodic signal the health of the heart or other organs can be monitored. Another way to assess a subject's health is to introduce a perturbation in the subject's response and measure the response to that change. An example is the oral glucose tolerance test, where the insulin response of a subject is tested by challenging the subject's body with a high glucose load. The rise and fall of glucose levels in the blood guides the diagnosis of diabetes.
Optical Coherence Tomography (OCT) is an optical backscatter technique, analogous to a sonogram, that is used to create high resolution images of tissues (several microns) at relatively shallow depths (a few mm). (“Optical Coherence Tomography (OCT): A Review”, J. M. Schmitt, IEEE Journal of Selected Topics in Quantum Electronics, July/August 1999, p. 1205). Recently, this technique has been applied to monitor blood glucose. (“Noninvasive Blood Glucose Monitoring With Optical Coherence Tomography, A pilot Study in Human Subjects”, K. Larin, et al., Diabetes Care, vol. 25, no. 12, December 2002, See also: R. O. Esenaliev et al., “Noninvasive Blood Glucose Monitoring With Optical Coherence Tomography”, Diabetes Care, Volume 25, Number 12, December 2002) and blood oxygenation (oximetry). (“Light absorption of (oxy-)hemoglobin assessed by spectroscopic optical coherence tomography”, D. J. Faber et al., Optics Letters, 2003, pgs. 1436-1438). An OCT method for brain oximetry is disclosed in U.S. Provisional Patent Application Ser. No. 60/485,761, “Method and Apparatus for Brain Oximetry”, filed by M. J. Schurman on Jul. 9, 2003. The 60/485,761 application is attached hereto.
In these sensor applications the change in the intensity of the reflected light from the tissue can be related to changes in the tissue being probed. Such changes can be due, for example, to variations in osmolyte concentrations, cell volume, or fluid volume surrounding the cells. The great strength of OCT sensors is that the small light/tissue interaction volume that they probe provides information on tissue changes at a highly localized level. Also, OCT measurements can be made continuously, thus providing a monitoring capability that was previously unavailable. One example of continuous monitoring, according to the prior art, is illustrated by FIGS. 1 and 2, which show, respectively, an OCT sensor signal from the skin and a blood glucose measurement as a function of time using standard assay techniques. In this test, a healthy subject was given a drink of glucose to induce an insulin response that serves to reduce the glucose levels in the subject. These tests are used routinely by doctors in the diagnosis of diabetes, however it is the macro-response of the human body over a period of three hours that doctors use to guide diagnosis.
It would be advantageous to have a method and apparatus to observe the response of the human body to this and other types of stimulus over shorter time scales.